In vitro culture of rat trophoblast is being used to establish predictable structure-function correlates affecting the development, differentiation and function of trophoblast. These parameters will be used to determine if these processes are autonomous or regulated. Rat trophoblast attaches to and displaces uterine epithelium and penetrates decidualized stroma by an invasive process to establish a relationship with maternal vasculature. Blastocysts, cultured in vitro, mimic in situ trophoblast providing a productive model for studying trophoblast development in experimentally defined conditions. Trophoblast is being characterized by analysis of specific developmental landmarks. Attachment and adhesion (surface glycoproteins) will be correlated with indices of invasiveness (plasminogen activator) and biochemical differentiation (hormone synthesis, secretion). Well characterized monolayers of individual endometrial cell types have been developed as a result of a new separation method. Qualitative and quantitative analysis of factors which influence the interaction between trophoblast and endometrial cells which modulates trophoblast expression is facilitated by analysis of trophoblast and endometrial cell monolayers under in vitro co-culture conditions.